The present invention relates to an improvement in the suspension polymerization of vinyl chloride. More particularly, the invention relates to an improvement in the method for the suspension polymerization of vinyl chloride to prepare a polyvinyl chloride resin having a relatively high bulk density and containing a remarkably decreased amount of fish eyes, in which the amount of polymer scale built-up on the reactor walls can be greatly decreased and the unreacted vinyl chloride monomer absorbed in the resin particles can be readily released to cause little problem of environmental pollution which badly affects the workers' health.
As is well known, polyvinyl chloride resins in general are industrially very important in respect of their excellent properties and relative inexpensiveness as a base resin of molding compositions for both of plasticized or flexible shaped articles and unplasticized or rigid shaped articles which are fabricated by various molding methods including calendering, extrusion molding, injection molding and the like. One of the important requirements for a polyvinyl chloride resin in the form of a powder is that the bulk density of the resin powder is as high as possible in order to save the volume capacity of the molding machine. This requirement is particularly important inter alia in the extrusion molding of unplasticized or rigid shaped articles of polyvinyl chloride resins.
Polyvinyl chloride resins are mostly prepared by the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium containing a dispersing agent dissolved or dispersed therein by agitation with a stirrer in the presence of a monomer-soluble polymerization initiator. Various attempts and proposals have been heretofore made with an object to obtain a polyvinyl chloride resin powder having an increased bulk density for the above mentioned reason including a method disclosed in Japanese Patent Kokai No. 59-168008 in which a portion of the monomer is retained and introduced into the polymerization mixture in the course of the polymerization reaction under proceeding, a method disclosed in Japanese Patent Kokai No. 57-7600 in which a specific polyvinyl alcohol of high degree of saponification is used as the water-soluble dispersing agent and so on.
These prior art methods are indeed not ineffective to give a polyvinyl chloride resin powder having a considerably increased bulk density. The polyvinyl chloride resin powder obtained by these prior art methods, however, is defective in respect of the low porosity of the particles and a relatively large amount of fish eyes contained therein. When the porosity of the resin particles is low, the unreacted vinyl chloride monomer absorbed in the resin particles is released only at a low velocity so that the content of the unreacted monomer in the resin product is necessarily high to cause a serious problem due to the toxicity of the vinyl chloride monomer such as pollution of the working environment in the processing of the resin product after the polymerization reaction and the molding process of the resin composition as well as a limitation in the application of the shaped articles of the resin composition.
Besides the above mentioned polyvinyl alcohol of a high degree of saponification, extensive tests have heretofore been undertaken for the performance of various types of polyvinyl alcohols as a dispersing agent used in the suspension polymerization of vinyl chloride including those insoluble in water but soluble in the monomer having a low degree of polymerization and low degree of saponification and those having various kinds of modifying groups available as a result of the progress in the preparation method of polyvinyl alcohols although the improvements obtained by using these specific polyvinyl alcohols can be far from satisfactory. For example, the monomer-soluble polyvinyl alcohol has a problem that the polyvinyl chloride resin powder obtained therewith has a rather decreased bulk density although some improvements can be obtained in the monomer releasability of the particles and the content of fish eyes in the resin powder. Modified polyvinyl alcohols are also defective because, when suspension polymerization of vinyl chloride is conducted by using such a dispersing agent, the amount of polymer scale deposition on the reactor walls is relatively large in addition to the problem of the poor uniformity in the particle size distribution of the resin powder due to the poor stability of the monomer suspension in the aqueous suspension medium.